In the past, a variety of piezoelectric power generating devices using the piezoelectric effect have been proposed. For example, the following Patent Document 1 discloses a piezoelectric power generating device 1001 illustrated in FIG. 13.
The piezoelectric power generating device 1001 includes a metal plate 1002 bent in a U-shape. At least one piezoelectric element 1003 is fixed to the metal plate 1002. One end of the metal plate 1002 is fixed to a fixing member 1004. The metal plate 1002 bent in a U-shape includes a first arm portion 1002a fixed to the fixing member 1004. The metal plate 1002 is bent at an end portion of the first arm portion 1002a opposite to an end portion of the first arm portion 1002a fixed to the fixing member 1004. Further, a second arm portion 1002c is connected to the first arm portion 1002a via a bent intermediate portion 1002b. The leading end of the second arm portion 1002c forms a free end.
The piezoelectric power generating device 1001 uses the metal plate 1002 bent in a U-shape as described above. When vibration or external force is applied from outside, therefore, the free end is substantially displaced, and is capable of generating a relatively large amount of power.
Further, the following Patent Document 2 discloses a piezoelectric power generating device using a piezoelectric power generating plate folded multiple times into an accordion shape. The piezoelectric power generating plate is a plate-shaped piezoelectric power generating element. Also in this case, with the use of the piezoelectric power generating plate folded into an accordion shape, the piezoelectric element is increased in area, and is capable of generating a large amount of power when vibration or external force is applied from outside.
Patent Document 1: Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2010-517285
Patent Document 2: Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2005-507627
In some fields using piezoelectric power generating devices, there are cases in which the vibration applied to a piezoelectric power generating device has a relatively low frequency. For example, in a piezoelectric power generating device attached to a motor vehicle or a piezoelectric power generating device which generates power by using vibration caused by human walking, the piezoelectric power generating device is required to generate power from vibration of relatively low frequency. To cause the piezoelectric power generating device to generate power from vibration of low frequency, it is necessary to reduce the resonant frequency of the piezoelectric power generating element.
In a case using a plate-shaped piezoelectric power generating element, therefore, it is necessary to increase the length of the plate-shaped piezoelectric power generating element or reduce the thickness thereof so as to reduce the resonant frequency of the piezoelectric power generating element. However, if the length of the plate-shaped piezoelectric power generating element is increased by folding the piezoelectric power generating element in a U-shape or by using a piezoelectric power generating element folded multiple times into an accordion shape, as described in Patent Document 1 or Patent Document 2, the height dimension is inevitably increased. Further, if the thickness of the plate-shaped piezoelectric power generating element is reduced, the strength of a folded portion is reduced, and plastic deformation or brittle fracture occurs. It is therefore difficult to reduce the thickness of the plate-shaped piezoelectric power generating element. Further, in the piezoelectric power generating device 1001 described in Patent Document 1, the piezoelectric element 1003 is attached only to a portion of the metal plate 1002. Therefore, the occupancy rate of the piezoelectric member in the piezoelectric power generating element is small, and thus the power generation amount is prevented from being sufficiently increased.
Further, in the piezoelectric power generating device described in Patent Document 2, the piezoelectric power generating plate having an accordion shape needs to be processed into a three-dimensional shape, and the processing is substantially complicated. Furthermore, an operation of performing a partial poling treatment on such a complicatedly shaped piezoelectric power generating plate in different directions is also complicated.